The performance of many modern electronic systems depends, to a large degree, upon the stable operation of solid-state integrated circuits in the system over a range of operating conditions. A stability factor with which many in the art are concerned is temperature stability, particularly the temperature stability of circuit response to input signals. More specifically, it is highly desirable that digital integrated circuits have stable switching characteristics over the expected temperature range, particularly in having a constant trip point voltage over temperature.
The integration of a large number of circuit elements onto a single integrated circuit "chip" has greatly improved circuit performance, due to the matching of device parameters obtained as a result of the simultaneous manufacture of the circuit elements on the same chip. Such matching reduces the sensitivity of the circuit to shifting in manufacturing parameters, as parameter variations will tend to similarly affect circuit elements on the same chip. However, sensitivity of circuit performance to manufacturing variability still exists, especially in sensitive circuits such as those used to sense whether an input signal is above or below a selected trip voltage.
By way of background, one type of conventional circuit for providing a stable reference voltage relative to an input voltage is the so-called Widlar band-gap reference circuit, an example of which is described in Gray, et al., Analysis and Design of Analog Integrated Circuits (John Wiley & Sons, 1977, 1984), at pp. 289-296. This circuit produces an output voltage that is equal to the Vbe.sub.(on) of a bipolar transistor plus a voltage proportional to the difference between the forward bias voltage drop across the base-emitter junction of two transistors, and thus generates a relatively stable reference voltage over temperature. Gray et al. also disclose an improved band-gap reference circuit having reduced sensitivity to power supply variations relative to the traditional Widlar circuit. These band-gap circuits are generally used to provide a temperature-insensitive reference voltage, for example as an input to a voltage regulator circuit.
It is an object of the present invention to provide a voltage level sense circuit in which the trip voltage remains stable over a temperature range.
It is a further object of the present invention to provide such a circuit in which the trip voltage is stable over process variations.
It is a further object of the present invention to provide such a circuit which may be implemented in CMOS-compatible technology.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.